This invention relates to air conditioning systems and particularly to systems employing both mechanical and natural cooling modes.
In modern air conditioning systems, efficiency and economy may be improved by utilizing the effects available with respect to outside air. In cooling systems particularly, natural cooling can be used when a demand for cooled air is initiated and when the outside air is cool enough and of a sufficient quality level which may be employed to satisfy that demand. By using natural cooling when possible and mechanical cooling at other times, a total system of greater economy and efficiency is achieved.
In a conventional automatic cooling system employing both natural and mechanical cooling systems, termed an "economizer system," a damper controlled vented ductwork is utilized. In a basic system, air is recirculated from the conditioned space into a mixing chamber. The mixing chamber also receives outside air from an outside air inlet. When cooling is demanded, and natural cooling can be employed, air is admitted from the outside air inlet through the mixing chamber into the space to be conditioned. When cooling is demanded and natural cooling is not available, then mechanical cooling is employed, principally on recirculated air. Outside air dampers are then closed, although partial admittance of outside air may be employed for fresh air ventilation.
The control systems conventionally employed for the foregoing described functions are of two basic types. In the first, a motor is employed to drive the damper in the outside air inlet duct. To control the motor at a fixed point and to prevent hunting or oscillation about the desired setting, a balancing potentiometer control system is employed. In such systems, the controller potentiometer and motor feedback potentiometer, together with a balancing relay, form a bridge circuit. As long as the value of the controlled medium remains at the controller set point, the circuit is balanced and the motor does not run. When the value of the controlled medium changes, the potentiometer wiper in the controller is moved. This unbalances the circuit and more current flows through one half of the balancing relay. The relay closes and runs the motor in the appropriate direction to correct for the imbalance. As the motor runs, the feedback potentiometer wiper moves to rebalance the circuit and stop the motor.
Thus, the motor will be activated in accordance with the settings of an appropriate controller and rapidly move to a position determined by the balancing of the potentiometer. The damper thus undergoes a series of rapid movements between successive fixed points determined by the controller settings and thereby modulates over a desired range, thus modulating the flow of outside air as desired. While this modulating action is desirable and effective, this requires the use of at least one relatively expensive motor and balancing relay control system.
In a control system of a second type, a slower moving motor without balancing relays has been employed to control outside air entry. In this case, a thermostat within the conditioned space demands cooling and a controller sets the outside air damper motor in either an on or off position. In this case, if natural cooling is available, the mechanical cooling will not come on and the outside damper will remain open until the space thermostat is satisfied. In this conventional system, the damper will regulate the outside air in response to the room thermostat and thus may create overshoot situations or at least permit the conditioned space temperature to oscillate about the desired set point by a wider margin.
It is the object of the present invention to provide an economizer system that will achieve the more precise control available with a modulating damper but without the use of expensive motors, balancing potentiometers, and balancing relays.
It is a further object of the present invention to enable a less expensive type of damper drive to be used with a control system that will provide a more precise control of the air temperature so as to provide a relatively narrower variation about a set point.